It is well known that EGR operation in an internal combustion engine, which is provided to decrease the amount of nitrogen oxides (NO.sub.x) in the engine exhaust, causes inferior flame propagation velocity in the combustion chamber of the engine. Therefore, it is necessary to advance the ignition timing during EGR operation in order to obtain the ignition timing required for maximum output power and maximum fuel comsumption efficiency. In this case it is necessary to advance the ignition timing in accordance with the "degree of EGR" (the ratio of the amount of the exhaust gas to be recirculated, to the amount of total gas supplied to the combustion chamber of the engine).
There has already been provided an apparatus for controlling the ignition timing of an internal combustion engine provided with an EGR system that comprises a vacuum advancer having at least one vacuum advance control diaphragm connected to the vacuum advance shaft of the distributor. A vacuum advance control chamber is formed on a side of the diaphragm remote from the distributor, and is connected to a port to which a flow control chamber of an exhaust gas recirculation valve (EGR valve) of the EGR system is also connected. In this known apparatus the ignition timing during EGR operation, in which a vacuum signal in said port is transmitted to the flow control chamber of the EGR valve for operating the valve, is advanced by said vacuum signal which is also transmitted to said vacuum advance control chamber of the vacuum advancer.
However, this known apparatus has the disadvantage that the ignition timing is too advanced when the engine is operating at a high rotational speed and, therefore, maximum output power and maximum fuel comsumption efficiency are not obtained. This is because: on the one hand, the degree of EGR is not constant with respect to the rotational speed of the engine (the higher the rotational speed, the lower the degree of EGR), and; on the other hand, the level of vacuum in the vacuum advance control chamber of the vacuum advancer is not substantially changed with respect to the rotational speed of the engine. Thus, the vacuum advance control chamber has too large a vacuum when the engine is operating at a high rotational speed. As a result, the vacuum advance shaft of the vacuum advancer is moved so that the ignition timing is too advanced during said operation with respect to the required ignition timing and, thus, the required ignition timing is not obtained.